1. Field of the Invention
The present invention relates to a mechanism for a bi-stable electrical relay and in particular a mechanism useful for overload relays such as are used to protect motors from overload or fault currents.
2. Background Art
Overload relays are specialized circuit breakers used with industrial motors to protect the motors from damages caused by overload or electrical faults. Typically, such motors are connected to a source of three-phase power through a contactor. In this case, the contactor is a heavy duty relay having three contact sets for breaking each of the three-phases of power upon movement of a yoke member within a contactor coil, the yoke member and coil together forming an electrical solenoid. The coil may be energized, and thus the contactor controlled, by current from a remote set of switches.
A contact of an overload relay is typically connected in series with the coil of the contactor to cause the contactor to open when an overload condition is sensed. The overload relay senses an overload condition by monitoring the current in each of the three-phases received by the motor windings. In the simplest case, the overload relay incorporates resistive heaters for each phase which are thermally coupled to one or more bi-metallic elements. An overload condition is indicated when the time integral of the motor current exceeds a predetermined value, the time integral being represented by the temperature of the resistive heaters. When an overload is sensed, the bimetallic switch opens, de-energizing the contactor coil and causing the motor to be disconnected from the line.
With advances in electronic circuitry, the bi-metallic element has been replaced with more complex circuitry. Such circuitry may sample current flow to the motor on a periodic basis and provide sophisticated overload prediction based not only on a simple thresholding but on more complex trend analyses. The output of this circuitry is typically a low-powered overload signal. In order for this overload signal to control the contactor coil current, a solid state switch may be required, adding to the complexity and cost of the overload relay.
Once "tripped" the overload relay remains in the open position and must be manually reset. Resetting is typically accomplished by a mechanical push button. When the reset push button is pushed, the contacts of the overload relay allow current to again flow through the contactor to the motor.
It is desirable that the connection of the reset button to the overload relay contacts be such that the contacts may open in the event of an overload even when the reset button is depressed. This prevents damage to the motor if an overload condition occurs or continues during a resetting of the overload relay, and more generally prevents the protective purpose of the overload relay from being defeated by a holding down or jamming of the reset button. This operability despite the pressing of the reset button is termed "trip free".
It is also desirable that the overload relay switch have a test button that allows testing of the contacts of the overload relay without the creation of an actual overload condition. When the overload relay switch is open by the test button, it should remain in the open position until it is reset, in a manner analogous as far as possible the opening caused by an actual overload condition.
The desire that an overload relay accept electrical overload signals such as from sophisticated overload detection circuitry, together with the requirements imposed by the reset and test buttons, and the requirement to make and break loads of as much as 600 volts, seems to demand a complex electromechanical mechanism with numerous parts, or an expensive all solid state device. Either alternative is expensive and may decrease the reliability of the overload relay.